The field of the invention relates to fuel vapor recovery systems wherein fuel vapors from the fuel system are inducted into an internal combustion engine. In particular, the invention relates to control of fuel vapor recovery in engines equipped with air/fuel ratio feedback control.
Air/fuel ratio feedback control is commonly used on modern motor vehicles to maintain air/fuel ratio near a desired air/fuel ratio such that efficiency of a catalytic converter is optimized. For example, when three-way catalytic converters (NO.sub.x, CO, and HC) are utilized, the inducted air/fuel ratio is maintained at a value which is within the catalytic converter's operating window. This value is commonly referred to as stoichiometry (14.7 lbs.air/1 lb.fuel).
Examples of known air/fuel ratio feedback control systems are disclosed in U.S. Pat. No. 4,641,623 issued to Hamburg and U.S. Pat. No. 4,763,634 issued to Morozumi wherein a desired fuel charge is calculated by dividing a measurement of inducted airflow with the desired air/fuel ratio. This desired fuel charge is then trimmed by a feedback correction value obtained from an exhaust gas oxygen sensor to maintain the desired air/fuel ratio.
Air/fuel ratio control is complicated by vehicles having vapor recovery systems. A typical vapor recovery system includes a vapor storage canister (usually containing activated charcoal) coupled to the fuel tank for adsorbing hydrocarbons which would otherwise be vented to the atmosphere. Since the canister has a finite storage capacity, it is necessary to periodically purge hydrocarbons from the canister. This is accomplished by a purge line connected between the canister and engine air/fuel intake. Under certain engine operating conditions, ambient air is purged through the canister and inducted into the air/fuel intake. In many fuel vapor recovery systems, vapors are also inducted directly from the fuel tank during a purge cycle.
Fuel vapor recovery systems create two general problems for feedback air/fuel ratio control. The induction of rich fuel vapors may exceed the range of authority of the feedback system. And, even when the air/fuel ratio feedback control system is capable of correcting for the induction of fuel vapors, the correction incurs a time delay before the perturbation in the inducted mixture propagates through the engine and exhaust to the exhaust gas oxygen sensor. During this time delay, perturbations in air/fuel ratio caused by induction of fuel vapors may go uncorrected.
The above problems have been addressed by U.S. Pat. No. 4,715,340 issued to Cook et al. More specifically, the rate of vapor flow is made proportional to airflow thereby reducing the perturbation in air/fuel ratio during a vapor purge. However, the inventor herein has recognized a disadvantage with this and similar approaches. More specifically, when throttle angle abruptly changes during a purge, a time delay is incurred before actual purge flow is increased in proportion to the increased inducted airflow. A lean perturbation in air/fuel ratio will occur during this time delay which is too rapid for correction by the air/fuel ratio feedback control system. Thus, every change in throttle angle may result in an uncorrected air/fuel ratio transient.